Matlab: find last non-NaN element and replace it with NaN

Question

How can I find for each row of the matrix in Matlab the index of the last non-NaN element and replace these values with NaN?

Thank you

Answer 1

This method may be an implementation. It uses a Logical_Array that indicates NaN values using "1" and non-NaN values using "0". Then each row using find() returns all the indices that are "0"/non-NaN number. By evaluating the maximum index using max() the last/greatest column can be retrieved for each respective row. Taking the max allows scenarios where the NaN values are scattered to be accommodated.

Matrix = [1 2 NaN 4 5;
         1 NaN 3 NaN 5;
         1 2 NaN NaN NaN];
     
[Matrix_Height,~] = size(Matrix);

Logical_Array = isnan(Matrix);

for Row = 1: +1: Matrix_Height
    
    Target_Row = Logical_Array(Row,:);
    [Minimum,Indices] = find(Target_Row == 0);
    Last_Non_NaN_Index = max(Indices);
    Matrix(Row,Last_Non_NaN_Index) = NaN;
    
end

Matrix

Result of Test Matrix:

Row 1: 5 → NaN
Row 2: 5 → NaN
Row 3: 2 → NaN

Ran using MATLAB R2019b

Answer 2

Consider x as:

x = [1   2   3   nan;
     3   4   nan nan;
     1   nan nan nan;
     nan nan nan nan]

You can obtain the index, for each row, of the first nan value with:

[~,ind]=max(isnan(x),[],2);

And then using sub2ind :

x(sub2ind(size(x),max(ind.'-1,1),1:length(x))) = nan

Or the one-liner:

x((circshift(isnan(x),-1)+isnan(x))>0)=nan